Linear power amplifiers have supply currents that are proportional to a signal envelope. If a supply current passes through a finite series resistance, it generates a supply modulation voltage that changes the actual supply voltage provided to the power amplifier dependent on the process signal envelope of a modulated signal. One case of supply modulation occurs when a power amplifier stage is biased by a regulator having finite output impedance. If an envelope current passes through the regulator, the envelope current creates a modulated voltage that adds to the actual supply voltage at a power amplifier stage. In single ended power amplifier configurations, the supply voltage is in series with the output signal voltage and thus any supply modulation appears as distortion on the signal path. Reducing the supply modulation level requires low regulator output impedance, which results in high power consumption within the regulator. As a result, power amplifier efficiency is reduced.
In the case of high electron mobility field-effect transistor (HFET) amplifiers having an inductive load, inductor series loss resistance will also generate a supply modulation voltage when passed by a power amplifier stage's envelope current. Reducing inductor series loss resistance results in large area inductors and thus high cost. One method used by the prior art to reduce supply modulation is to use differential amplifier configurations. Any supply modulation voltage appears as a common mode voltage and is thus highly rejected when transferring to a differential output. The main drawbacks of this technique are size and cost penalties brought by a differential architecture.
Another so called brute-force technique used for prior art linear power amplifiers reduces output impedance at a regulator output through the use of a stronger feedback loop. Such techniques result in higher current consumption in the regulator, which degrades overall power amplifier efficiency performance.
Thus, a need exists for alternative supply modulation mitigation solutions that maintain high power amplifier efficiency. However, such alternative supply modulation mitigation solutions must not significantly impact die area and/or cost of the power amplifier.